Leads for electrical stimulation of a spinal cord, particularly those employed to induce paresthesia in areas of the body experiencing pain, are known in the art. Epidural electrical stimulation of the spinal cord has been shown to be effective in relieving certain types of pain. One approach to providing such stimulation is to introduce a transcutaneous lead directly into the epidural space overlying the spinal cord. Such leads may be chronically implanted into a patient adjacent to the spinal cord to provide continuous treatment for pain.
Over time, and particularly during the first two or three weeks following lead implant surgery, the position of alead with respect to the spinal cord may inadvertently change. Because of such displacement, the lead may not provide stimulation to the original target spinal area. Such positioning changes may impair the clinical benefit of the stimulation provided by the lead. One method disclosed in U.S. Pat. No. 3,636,940 to Timm et al. involves suturing the lead in place after implantation. Suturing adds to the time required to perform surgery and may have adverse effects on the implant site.
Selection of the stimulation site within the spinal cord is performed at implantation. The surgeon generally places the electrode either surgically or percutaneously at the exact position to be stimulated. The position is determined by providing test stimulations with the lead. Leads placed in a less than optimal position may contribute to disagreeable sensations or unintended involuntary motor responses in the patient. Thus, time and effort are necessary to place existing leads in the desired region of the spinal cord during implantation. Often, it is even more difficult and time-consuming to re-adjust leads that have migrated once the leads have been implanted in a patient's body. Moreover, leads that have migrated after surgery are also susceptible to causing side effects such as disagreeable sensations or unintended involuntary motor responses. In extreme cases, repositioning of leads using further surgery is required.
To limit the need for physically repositioning chronically implanted leads, some techniques have been developed to compensate for changing post-operative lead positions. One technique is to electronically select one or more electrodes to generate a pulse signal to tailor or direct the resulting stimulation pattern to a particular individual's painful body part site or region. Those methods can partly overcome the effects of small lead migrations and may minimize unwanted motor responses.
Despite the existence of such methods of compensation and correction, however, unpleasant physical sensations and reduced efficacy of stimulation can still result from lead migration.
A percutaneous lead designed to be implanted adjacent the spinal cord is discussed in U.S. Pat. No. 4,285,347 issued to Hess. The Hess lead employs a looped protrusion at the distal tip to stabilize the electrode's position. U.S. Pat. No. 5,121,754, and assigned to Medtronic, Inc., also provides a percutaneous lead with a deformable distal shape. The distal portion is held straight with a stylet during insertion into the epidural spinal canal. After placement, the stylet is removed and the bend deforms within the epidural space. Both of these leads are known to post-operatively migrate, however, to some extent, are shaped and configured at the time of implantation.
Other percutaneous leads and devices and methods employed include those disclosed in the U.S. Patents listed below in Table 1.
Table 1
U.S. Patent No. Title
U.S. Pat. No. 5,733,322 Positive fixation percutaneous epidural neurostimulation lead PA0 U.S. Pat. No. 5,255,691 Percutaneous epidural lead introducing system and method PA0 U.S. Pat. No. 5,205,828 Epidural needle location indicator assembly PA0 U.S. Pat. No. 5,119,832 Epidural catheter with nerve stimulators PA0 U.S. Pat. No. 5,081,990 Catheter for spinal epidural injection of drugs and measurement of evoked potentials PA0 U.S. Pat. No. 5,058,584 Method and apparatus for epidural burst stimulation for angina pectoris PA0 U.S. Pat. No. 4,808,157 Multi-lumen epidural-spinal needle PA0 U.S. Pat. No. 4,467,800 Tool for creating a pocket for a epidural electrode PA0 U.S. Pat. No. 4,383,532 Epidural lead advancer PA0 U.S. Pat. No. 4,141,365 Epidural lead electrode and insertion needle
The stimulation systems of U.S. Pat. Nos. 5,501,703; 5,628,317; 5,643,330, and 5,713,922 assigned to Medtronic, Inc. disclose a simplified surgical lead resembling a paddle as shown in cross-section in FIGS. 7 and 8. Those figures illustrate that such a paddle-like lead may tend to slip from its optimal or original position subsequent to implantation. Such slippage from the midline can occur because of several factors, including, general lead migration (as described above), lateral positioning during surgery, or an asymmetrical position of the patient's spinal cord within the canal. Adaptation of the lead itself to withstand lead migration or for aberrant positioning after implantation would therefore be beneficial.
As those of ordinary skill in the art will appreciate readily upon reading the Summary of the Invention, Detailed Description of the Preferred Embodiments and Claims set forth below, at least some of the devices and methods disclosed in the patents of Table 1 and referenced elsewhere herein may be modified advantageously in accordance with the teachings of the present invention.